In the context of this disclosure, a "multichannel optical imaging system" has a plurality of spatially parallel optical channels to supply a like number of more or less independently modulated optical stimuli for writing images on a photosensitive recording medium. There are known optical image bars and multispot laser scanners which respond to this definition. Furthermore, the definition embraces imaging systems which have a plurality of distinct optical recording beams, as well as systems of the type in which a single beam is spatially modulated to provide a plurality of generally parallel optical stimuli.
The parallelism of multichannel optical imaging systems is attractive, especially for high speed printing. As a general rule, image bars are oriented so that their optical channels are spatially displaced from each other in the tangential direction at the image plane (i.e., transversely with respect to the imaging field) for line printing and similar applications. Multispot raster output scanners (ROS's), on the other hand, typically are oriented so that their optical channels are offset from each other in the sagittal direction (sometimes referred to as the "process" direction) at the image plane. Multispot ROS's usually have only a few channels, but optical image bars often have many.
Despite their functional and structural differences, most multichannel optical imaging systems perform best when their channels are well balanced optically. Significant, unintended differences between the nominal (i. e., unmodulated) optical illumination or optical transmission characteristics of the channels of such an imaging system are a potential source of unwanted imaging defects, such as streaks. Unfortunately, however, various factors, such as ordinary electrical and optical drift, as well as the normal accumulation of dust and other environmental contaminants, tend to upset the channel balance of these imaging systems as a function of time. The aforementioned copending application provides a general solution to that problem, but it does not deal with the associated problem which arises when there is significant and unavoidable optical crosstalk among the optical channels of such an imaging system, whereby adjustments made to the nominal optical output intensity of one channel or group of channels affect the nominal optical output intensities of its neighbors.